The present invention relates to a reamer assembly, and in particular to a multi-stage reamer assembly providing a frustro-conical array of individual reamer teeth.
In urban areas, it is often necessary to place a production casing for telephone lines, water mains and the like under existing road surfaces. The customary method for providing an underground hole large enough to contain such a casing involves several steps. First a small diameter pilot hole is drilled along the desired path of the casing with a drill having a trailing drill string. Then, a reamer having a slightly larger diameter than the pilot hole is attached to the drill string and run through the pilot hole to enlarge it. After the first reamer pass, a slightly larger reamer is attached to the drill string and used to further enlarge the hole. This process is repeated until the hole has reached the desired diameter. With this method, the reamers can either be pulled or pushed through the hole by the drill string on each pass.
In the customary method for enlarging a pilot hole to the desired diameter to accommodate a casing, the drill string is ordinarily chosen to have a diameter slightly less than the pilot hole bored by the drill bit. When successively larger reamer stages are added, the same drill string is used for convenience and the diameter of the drill string is not increased. As a result, when successively larger reamers are pulled through the pilot hole, the drill string is not flush with the sides of the hole and does not provide a guide for the reamer. The reamer will follow the path of the least resistance around rocks and other obstructions and the reamed hole will not be coaxial with the pilot hole and can have curves and bends along its length. If the reamer is pushed through the pilot hole, the drill string provides no guide whatsoever and again the reamer will follow the path of least resistance. An added difficulty with pushing a reamer through a hole to enlarge it is that the relatively thin drill string is potentially subject to columnar failure especially when larger reamers are used.
In urban areas wherein a large number of such conduits are often placed in a limited area, exact placement of the production casing between other casings already in place is often required, and this exact placement cannot be achieved with customary tools and methods for forming such holes. Furthermore, the hole will often be curved making it difficult to place a straight production casing in the hole. Hence, while such tools and methods are often used in the placement of pipelines or other conduits, a variety of difficulties usually arise in the course of such use.
The use of multi-stage reamers has recieved little attention in the prior art. A variety of multi-stage drills are illustrated in the prior art, such as those illustrated in the patents of Kandle, U.S. Pat. No. 2,780,439, and the patent to Seidlmayr, U.S. Pat. No. 2,815,935. However, these drills have a corkscrew-shaped surface which is used to pass the drilling earth along the axis of the drill as the drill is being used to bore a hole. As a result, these drills provide a solid surface spanning the entire diameter of the hole when viewed axially. This configuration is acceptable in drilling a fresh hole such as a pilot hole, but cannot be readily adapted for use as a reamer because clays commonly found in many areas will ball up in front of the reamer to fill the pilot hole and block the progress of the reamer. As a result, multi-stage drills known in the prior art have not and cannot be efficiently adapted for use as a reamer.